(1) Field of the Invention
The invention is related to a flexbeam unit for a multi-blade rotor of a rotary wing aircraft, said flexbeam unit comprising a plurality of flexbeam elements with the features of claim 1.
(2) Description of Related Art
A flexbeam unit is used in a so-called hingeless or a so-called hinge- and bearingless rotor system to connect rotor blades of a multi-blade rotor of a rotary wing aircraft to an associated rotor shaft of the aircraft. During operation, i.e. rotation of the multi-blade rotor, the flexbeam unit must withstand tremendous centrifugal forces that the rotor blades apply thereto, while permitting their flapping, pitch and lead/lag motions. Therefore, the flexbeam unit comprises special, in particular fiber reinforced composite material flexbeam elements that are flexible enough in torsion to allow twisting for blade movement without bearings, in the case of a bearingless rotor system, or without discrete hinges, in the case of a hingeless rotor system.
These flexbeam elements usually possess lead-lag-soft regions that permit motions of associated rotor blades in a hingeless or a hinge- and bearingless rotor system in the lead-lag direction. The lead-lag-soft regions thus constitute fictitious vertically oriented axes, so-called virtual lead-lag hinges, about which the rotor blades execute forward and backward lead-lag motions. Furthermore, these flexbeam elements realize flapwise-soft regions that enable flapping of the associated rotor blades in the vertical direction and, thus, constitute fictitious horizontally oriented axes, so-called virtual flapping hinges, about which the associated rotor blades execute upward and downward flapwise motions in a hingeless or a hinge- and bearingless rotor system. The distance between these virtual flapping hinges and the axis of the rotor shaft is referred to as the flapping hinge distance. Moreover, only in a bearingless rotor system, these flexbeam elements usually comprise torsion weak regions.
In a hingeless or a hinge- and bearingless rotor system, the flapping hinge distance is relatively large. The flapping hinge distance is usually approximately 7 to 12% of a given rotor-disc radius, measured from the axis of the rotor shaft radially outward to a tip of a selected rotor blade. A large flapping hinge distance in a hingeless or a hinge- and bearingless rotor system allows, during operation, for a good control response and maneuverability of the rotary wing aircraft, but unfortunately also leads to a high natural flapping frequency. This relatively high natural flapping frequency and the vibrations that result therefrom in the hingeless or the hinge- and bearingless rotor system are disadvantageous in terms of flying characteristics of the rotary wing aircraft and lead to large stresses on blade connectors provided at the flexbeam elements for connection of the associated rotor blades. Because of these large stresses and the strength that is therefore required for the flexbeam elements, it is extremely difficult to reduce the virtual flapping hinge distance or decrease it below the specific value of about 7%.
The document EP 1 613 534 B1 describes a multi-blade rotor having four rotor blades, wherein each two rotor blades are located exactly opposite to one another with a 180° offset and wherein very flatly designed flexbeam elements associated with these rotor blades are embodied continuously, i.e. with no interruption, and attached to an associated rotor star. The flexbeam elements are realized as connector arms, each connecting a pair of rotor blades, i.e. two diametrically opposed rotor blades. More specifically, each pair of rotor blades is connected via two parallel connector arms that pass an associated rotor shaft on diametrically opposed passing points in a tangential manner, such that the pair of rotor blades and the two connector arms form an oval-shaped opening. As a result, centrifugal-force segments formed by these connector arms are likewise continuous so that the considerable centrifugal forces applied by one of the rotor blades of a given pair of rotor blades can directly be transferred to the diametrically opposed rotor blade. Furthermore, each pair of rotor blades possesses two common auxiliary flapping hinges and a common or different virtual lead-lag-hinge. Accordingly, a flapwise-soft connection of the rotor blades is produced by the fact that the connector arms can deflect between auxiliary flapping hinges, thus, leading to a greater softness in the connection region.
While the realization of flexbeam elements in the form of such connector arms allows to reduce the virtual flapping hinge distance to values of less than 7%, these connector arms are complicated in manufacturing because of their reduced cross section and their very flat design, which however need to allow for a predetermined weakness in torsion, so that they only have a comparatively reduced fatigue lifetime. The reduced cross section and very flat design also complicate the attachment of the connector arms to the associated rotor star which should not weaken the overall strength of the connector arms, i.e. the attachment, but still allow for the predetermined weakness in torsion. Therefore, multi-blade rotors having flexbeam units comprising such connector arms are not suitable for comparatively big rotary wing aircrafts where the loads that need to be lifted by the multi-blade rotors trespass a given threshold.
The document EP 0 538 088 A1 describes a multi-blade rotor, especially for a helicopter rear anti-couple propeller, which includes a hub body to which are connected twistable straps. Each such twistable strap is formed by two bundles of fibers with a flattened cross section that are arranged radially around the hub. Each one of the bundles forms at least two halves of different blades and is twisted and curved in the region of its passage near the center of the hub body in such a way that it forms a tangent to the circumference of the center of the hub body along the greater dimension of its cross section. Furthermore, each one of the bundles overlaps a first other one in the circumferential direction of the multi-blade rotor immediately following bundle and is overlapped by a second other one in the circumferential direction of the multi-blade rotor immediately preceding bundle. However, each such bundle is twisted in the region of its passage near the center of the hub body, thereby decreasing its stiffness such that this multi-blade rotor as such is not suitable for use with comparatively big rotary wing aircrafts.